The present invention generally relates to an image forming apparatus using an electrophotographic method, and more particularly to an image forming apparatus in which an image forming onto a photosensitive medium is performed by means of laser beam, and in which a stepping motor is used for driving a photosensitive medium and conveying rollers.
As an image forming apparatus using an electrophotographic method, a laser printer, a digital copying machine and a facsimile machine are known. A laser printer, for example, performs a formation of an image onto a photosensitive medium by radiating a laser beam onto the photosensitive medium, the laser beam being controlled based on image data supplied by a host computer. The photosensitive medium is normally formed in a drum shape, and is rotationally driven by a motor. Scanning along a direction perpendicular to a rotational direction (primary scanning direction) of the photosensitive drum in an image forming operation is normally performed by deflecting a laser beam which is reflected by a polygon mirror rotating at high speed. A secondary scanning is performed by means of a rotation of the photosensitive drum. In the secondary scanning a position of a laser beam spot on the photosensitive drum is moved along the rotational direction of the photosensitive drum.
In the above-mentioned image forming apparatus, in order to obtain a high line-density of an image formed on the photosensitive drum, a high accuracy of driving of the photosensitive drum is required. Thus, use of stepping motors has increased. The reason for using a stepping motor is that a rotational speed and an angular position of a rotational shaft of a stepping motor can be controlled by an open loop control without a position detector or a feedback circuit. This means that using a stepping motor brings an advantage that a highly accurate control of rotation of the photosensitive drum can be performed using a simple construction. Additionally, a stepping motor has a quick response due to having a pulse drive. Further, the cost of stepping motors has been reduced.
A recording paper fed to a photosensitive drum is conveyed at the same speed as a circumference speed of the photosensitive drum because an image formed by toner on the photosensitive drum is transferred to the recording paper. Accordingly, when a stepping motor is used for driving a photosensitive drum, a stepping motor is also used for driving a conveying mechanism for recording papers.
In a conventional image forming apparatus, since a process speed during an image forming operation is set to be constant, a time needed for forming an image of one page depends on a size of a recording paper being used. For example, a time needed for recording on sheets of the same size is constant regardless of whether an image is formed only in a short length part of an entire recording paper or on a long part thereof. In other words, an excessive time is spent in forming an image for one page in a case where the image is not formed over the entire recording paper lengthwise. Therefore, there is a problem in that a constant conveying speed of a recording paper prevents a reduction of an overall image forming process time.
Further, in a conventional image forming apparatus, alteration of a recording density in the primary scanning direction is performed without changing a scanning speed in the secondary scanning direction, that is, the alteration is performed by changing a rotational speed of a polygon mirror. When increasing recording density, for example, an increase of recording density in the secondary scanning direction can be performed by increasing a rotational speed of the polygon mirror, and an increase of recording density in the primary scanning direction can be performed by increasing a number of switching of the laser beam. The switching of the laser beam is performed by shortening a cycle of clock pulse supplied to the laser generator. Additionally, if the rotational speed of the polygon mirror is increased, energy of the laser beam per unit area irradiated onto the photosensitive drum is reduced, and thus the intensity of the laser beam must be increased. Accordingly, the cycle of a clock used for controlling an irradiation of a laser beam and an output intensity of the laser beam must be changed due to the change in polygon mirror rotating speed; thus there is a problem in that a control of the image forming operation becomes complex.